1. Field of the Invention
The present invention relates to rotatable equipment such as digger derricks. In particular, the invention relates to a rotational float system that may be selectively operated to permit a digger derrick boom to freely rotate toward a side load to simplify operation of and prevent damage to the digger derrick.
2. Description of the Prior Art
Digger derricks, cranes, and other types of rotating equipment are commonly used to perform many different jobs, including digging holes for utility poles and installing screw anchors for supporting the poles. Digger derricks are typically mounted to mobile utility vehicles and include a rotating turntable from which a boom extends. The boom may be raised, lowered, extended, and retracted to position its outboard end in various locations to perform digging or hoisting operations. Digger and auger assemblies may be coupled with the outer end of the boom for digging holes for utility poles and for installing screw anchors. Once installed, anchors are typically connected to guy wires to stabilize the top portion of a pole in a particular direction and therefore are preferably installed into the ground at some angle relative to vertical.
Digger derricks and other rotating equipment are often subjected to excessive and undesirable side loading. Side loading can be applied to a digger derrick boom when, for example, a winch attached to the boom is used to pull a heavy load in from the side of the boom. Side loading also often occurs when an auger-type digger coupled with the boom “corkscrews” into the ground due to the application of excessive pressure in driving the auger or when anchors are improperly installed. Excessive side loading can cause damage or destruction of the digger derrick's boom, turntable, and rotation drive mechanism. Improper installation of an anchor can also significantly reduce its pull-out strength.
Special care must be taken when installing screw anchors to avoid or minimize such side loading. When a screw anchor is to be installed directly behind a truck to which a digger derrick is mounted (so that the truck is in-line with the direction of the anchor), it is necessary for the digger operator to monitor and control at least three functions to ensure that the boom generally follows the path of the anchor. Specifically, the operator must simultaneously operate the digger motor, lower the boom, and retract the boom as the anchor is being driven into the ground. If an anchor is to be installed on either side of the truck, as is common with the majority of roadside pole installations, it is also necessary for the digger operator to rotate the turntable and the boom toward the anchor as the anchor enters the ground to correctly follow the path of the anchor. Those familiar with the operation of digger derricks will appreciate that simultaneously monitoring all of these movements requires a great deal of skill and training. If an operator forgets or otherwise fails to correctly follow the path of an anchor into the ground by manipulating the digging, boom extending/retracting, boom lowering/raising, and rotating functions, the anchor will tend to pull the boom to one side, exerting considerable side loading on the derrick and/or reducing the pull-out strength of the anchor.
Systems have been developed for protecting digger derricks and other rotating equipment against excessive side loads. U.S. Pat. No. 4,100,973 (the '973 patent), hereby incorporated into the present application by reference, discloses a side load protection system that attempts to sense undesired side load levels and respond with appropriate action when pre-established limits have been exceeded. The system uses relief valves that open when pressure increases beyond a desired level in a hydraulic motor to allow the boom driven by the motor to slip or rotate toward the external load, thereby reducing the torque of the side load. Other side load protection systems are being developed that shut down certain digger derrick operations once side load limits have been reached.
Although generally effective, existing side load protection systems are subject to misuse. For example, operators who are aware of side load protection systems may intentionally neglect to rotate a digger derrick's boom to follow an anchor into the ground, knowing that the side load protection system will permit the turntable and boom to slip or rotate toward the side load once an excessive amount of side load has been exerted on the boom. This practice should be discouraged because existing side load protection systems require application of a high degree of side loading before they are activated. Such side load protection systems therefore are intended to provide protection from occasional high side loads, not repeated side loads resulting from everyday operation such as the installation of screw anchors. If an operator routinely relies on a side load protection system during normal operation of a digger derrick, the cumulative effect of the side loading that occurs before the side load protection system is engaged can significantly damage the digger derrick and reduce the holding power of any installed screw anchors.